不良・失敗
blackening
Japanese: 黒化
A phenomenon in which metallic lead precipitates when lead-containing glass is fired with sulfate-containing plaster, resulting in black spots or black streaks. / Note: This can be prevented by using lead-free plaster or by limiting the firing temperature to 880°C.
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Blackening is a phenomenon in which PbS and metallic lead (Pb⁰) precipitate when lead-containing glass is fired with sulfate-containing refractory gypsum, causing black spots or black streaks to appear in the glass. This process is more likely to occur in the presence of reducing agents such as residual carbon or organic matter.
This can be prevented by using lead-free plaster or by limiting the firing temperature to 880–900°C.
This entry is written in Japanese by Rei Chikaoka and translated for readers outside Japan. If something reads oddly, the original is the source of truth.